1. Field of the Invention
This invention is directed to ohmic contacts for semiconductor devices and more particularly, to an ohmic contact of the AuGeNi system onto a GaAs substrate.
2. Description of the Prior Art
Interest in developing GaAs based semiconductor devices such as high speed low power field effect transistors, has been increasing in recent years. One of the important requirements necessary in developing these devices is the fabrication of low resistance and thermally stable ohmic contacts with uniform interfaces. The AuNiGe system has been extensively used as the ohmic contact to GaAs materials that yield reproducible contacts with low contact sensitivity. However, the spread of the resistance values of an ohmic contact is usually large which could seriously limit the device performance.
The spread of contact resistance is believed to be closely related to the non-uniform microstructure at the interface. Ge containing protrusions, which are believed to be the paths of major current flow, are generally formed after contact alloying and cause a rough interface. Large spread of the contact resistance is probably due to such interface complexity, especially if the non-uniformed distribution is near the contact edges where current crowding is most important. Improved uniformity of the interfacial microstructure of the contacts is expected to reduce the spread of contact resistance.
Thermal instability is another major concern for the AuNiGe ohmic contacts. After formation, the contacts are required to retain their properties after high temperature cycling required to complete the device and circuit fabrication. Poor thermal stability of the contact materials results in a spread of the contact resistance due to a deterioration of the microstructure required for low contact resistance and optimum device performance.
Improved uniformity of the ohmic contact has been reported when a sputter cleaning of the GaAs substrate was done before the deposition of the metals. A. Callegari, et al., Appl. Phys. Lett. 46, 1141 (1985). In addition, the deposition of a 5 nm of Ni as a first layer of AuNiGe ohmic contacts significantly reduces the spread of the contact resistance as well as the mean value of contact resistance as explained by M. Murakami, et al., J. Vac. Sci. Technol. B4, 903 (1986).
One method of counteracting the problem of non-uniformity in the metal GaAs interface is shown in U.S. Pat. No. 3,959,522 in which a layer of gold is deposited onto a heated semiconductor body, depositing tin onto the cooled body and then alloying the two layers to the semiconductor material. The tin uniformly alloys itself to the gold layer and penetrates into the semiconductor material to form a uniform contact with the semiconductor material which reduces the problem of the tin balling up on the surface. The deposition of an initial layer of gold onto a GaAs semiconductor material is also shown in U.S. Pat. No. 4,179,534 in which initial layers of gold and tin are deposited at temperatures below 200.degree. C. and then alloyed to the semiconductor material which results in the ohmic contacts having high peel strength. None of the above methods obviate the problem of the spread of contact resistance after high temperature annealing.